1. Field of the Invention
This invention relates to a display device such as a liquid crystal display device, EL (Electroluminescent) display device, plasma display device, etc. and a method for manufacturing the same.
2. Description of the Related Art
The above display device will be explained taking an example of a semi-transmissive liquid crystal display device (JP-A-2004-212532).
FIGS. 4A to 4F is a sectional view showing an example of a method for manufacturing a TFT (Thin-Film-Transistor) substrate employed for a conventional semi-transmissive liquid crystal display device. In FIGS. 4A to 4F, reference numeral 1 denotes a first metallic thin film, 2 a first insulating film, 3 a semiconductor active film, 4 an ohmic contact film, 5 a source electrode, 6 a drain electrode, 7 a second insulating film, 8 an organic film, and 9 a transparent conductive film. Reference numerals 10, 11 denote a third metallic film. The metallic film 10 is formed beneath the metallic film 11. Reference numeral 12 denotes one of contact holes.
Referring to FIGS. 4A to 4F, an explanation will be given of a method for manufacturing the TFT substrate. First, the first metallic thin film 1 is formed on a glass substrate by e.g. sputtering. Subsequently, in a first photolithography process, gate wirings, a gate electrodes and a gate terminals are formed. The first metallic thin film may be made of Cr, Mo, Ta, Ti, Al, Co, an alloy of one of these elements and a minute quantity of the other material added thereto, or a laminated film of these materials. In this process, a pattern of first metallic films 1 is formed on the substrate as shown in FIG. 4A.
Next, by plasma CVD, the first insulating film 2, semiconductor active film 3 and ohmic contact film 4 are successively deposited. The first insulating film 2 may be made of SiNx, SiOy, etc. which is used as a gate insulating film. The semiconductor active film 3 may be an amorphous silicon (a-Si) film or a polysilicon (p-Si) film. The ohmic contact film 4 may be an n-a-Si film or an n-p-Si film which is the a-Si film or the p-Si film doped with a minute quantity of e.g. phosphorus (P). Next, in a second photolithography process, the semiconductor active film 3 and ohmic contact film 4 are patterned in a region where at least a TFT part is to be formed. Thus, the structure as shown in FIG. 4B is formed.
Next, a second metallic thin film is deposited by e.g. sputtering. The second metallic thin film may be made of Cr, Mo, Ta, Ti, Al, Co, an alloy of one of these elements and a minute quantity of the other material added thereto, or a laminated film of these materials. Next, in a third photolithography process, the second metallic film is formed into the source electrode 5 and the drain electrode 6. Thereafter, the ohmic contact film 4 is etched. Through this process, a central region of the ohmic contact film 4 of the TFT part is removed so that the semiconductor active film 3 is exposed. Thus, the structure as shown in FIG. 4C is formed on the substrate.
Further, after the second insulating film 7 and the organic film 8 have been formed, these films are patterned in a fourth photolithography process. In a later step of this process, the contact holes 12 are made. Thus, the structure as shown in FIG. 4D is formed.
Further, the transparent conductive film 9 is formed and patterned in a fifth photolithography process. The transparent conductive thin film 9 may be a transparent conductive film of e.g. ITO (Indium Tin Oxide). Thus, the structure as shown in FIG. 4E is formed. Thereafter, the third metallic film 10, 11 is formed and patterned in a sixth photolithography process. Thus, the structure as shown in FIG. 4F is formed. The transparent conductive thin film 9 and the third metallic thin film 10, 11 serves as a pixel electrode for driving the liquid crystal display device. The part in which the transparent conductive thin film 9 is exposed is a transmitting part which transmits the light from backlight. The part in which the third metallic thin film 10, 11 is provided is a reflecting part which reflects external light.
The TFT array substrate made as described above is bonded to a CF (Color Filter) substrate equipped with opposite electrodes. Liquid crystal is injected in between these substrates. The resultant structure is placed on a light emitting side of a planar light source device. Thus, the semi-transmissive liquid crystal display device is manufactured.
Incidentally, there is disclosed a technique similar to the technique for stacking or layering a metallic film on a transparent conductive thin film limited by the means for solving the problem described later (JP-A-2000-89247), which will be described later.
Meanwhile, the semi-transmissive liquid crystal display device described above provides a fear of generating a display defect. In the course of an increasing demand for large-scaling and enhanced definition of the liquid crystal display device, suppressing this display defect is a very important problem.
The main cause leading to the above display defect is as follows.
The surface of the organic layer 8 stacked on the substrate actually is not flat but has a uneven shape. Therefore, the transparent conductive thin film 9 also cannot be stacked flatly on the organic layer 8, but actually provides an uneven shape and pin holes.
The third metallic thin film 10, 11 will be stacked on the transparent conductive thin film 9 with the unevenness and pinholes. And during the etching process of the third metallic thin films 10, 11, an etchant will soak into the pin holes or a region with a thin thickness of the transparent conductive thin film 9. Thus, the transparent conductive thin film 9 itself and the first and second metallic films beneath it will be etched so that the malformation of these films occurs. This leads to the display defect due to a wire break.
In the above description, by way example, the transparent conductive thin film 9 stacked on the organic layer 8 was explained. However, it is not true that the problem of flatness in the stacked layer is attributable to only the organic layer 8. The transparent conductive thin film stacked on the stacked layer with no evenness also suffers from the same problem. Further, although the problem to be solved was explained for the semi-transmissive liquid crystal display device, the same problem will occur in a general display device.